1. Field of the invention
The present invention relates to a thermal printer with a thermal head for line-wise heating a heat-sensitive sheet to produce an image, in particular an image on a transparent support for medical diagnostic purposes.
2. Description of the Prior Art
Thermal imaging or thermography is a recording process wherein images are generated by the use of image-wise modulated thermal energy.
In thermography two approaches are known
1. Direct thermal formation of a visible image pattern by the image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. PA1 2. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an image-wise heated donor element into a receptor element.
A survey of "direct thermal" imaging methods is given in the book "Imaging systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), Chapter VII under the heading "7.1 Thermography".
Common thermal printers comprise a rotatable drum and an elongate thermal head which is spring-biased towards the drum to firmly line-wise contact a heat-sensitive material which is passed between the head and the drum.
The thermal head includes a plurality of heating elements and corresponding drivers and shift registers for these elements. The image-wise heating of a sheet is performed on a line by line basis, with the heating resistors geometrically juxtaposed along each other in a bead-like row running parallel to the axis of the drum. Each of these resistors is capable of being energised by heating pulses, the energy of which is controlled in accordance with the required density of the corresponding picture element.
In thermal dye transfer the sheet, i.e. the image receiving sheet, is attached to the rotatable drum, and a dye donor sheet or web is conveyed by frictional contact with the rotating sheet past the thermal head.
In direct thermal image formation, a single heat-sensitive sheet is conveyed between the thermal head and the drum, and the image is directly produced in the sheet. The sheet is not attached to the drum but is advanced between the head and the drum by frictional contact of its rearside with the drum.
We have found that if the sheet transport during printing occurs by frictional contact of the rearside of the sheet with the driven print drum, only at the place of the thermal head, control of the actual speed of advance may be insufficient so that the quality of the printed thermal image may become unsatisfactory.